1. The Field of the Invention
Embodiments of the present invention relate generally to x-ray devices. More particularly, embodiments of the present invention relate to anodes.
2. The Related Technology
The x-ray tube has become essential in medical diagnostic and inspection imaging, medical therapy, and various medical testing and material analysis industries. Such equipment is commonly employed in areas such as medical and industrial diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis.
An x-ray tube typically includes a vacuum enclosure that contains a cathode assembly and an anode assembly. The vacuum enclosure may be composed of metals, glass, ceramic, or a combination thereof, and is typically disposed within an outer housing. At least a portion of the outer housing may be covered with a shielding layer (composed of, for example, lead or a similar x-ray attenuating material) for preventing the escape of x-rays produced within the vacuum enclosure. In addition, a cooling medium such as a dielectric oil or similar coolant, can be disposed in the volume existing between the outer housing and the vacuum enclosure in order to dissipate heat from the surface of the vacuum enclosure. Depending on the configuration, heat can be removed from the coolant by circulating the coolant to an external heat exchanger via a pump and fluid conduits. The cathode assembly generally consists of a metallic cathode head assembly and a source of electrons highly energized for generating x-rays. The anode assembly, which is generally manufactured from a refractory metal such as tungsten, includes a focal track that is oriented to receive electrons emitted by the cathode assembly.
The anode assembly in some x-ray tubes includes a rotating anode comprising a solid disk target, an annular focal track located near the outer perimeter of the target, and an anode hub formed at the center of the anode for mounting the anode to an anode support assembly inside the vacuum enclosure. The anode support assembly includes, among other things, a support shaft and a rotating bearing assembly supporting the anode.
During operation of the x-ray tube, the anode is rotated and the cathode is charged with a heating current that causes electrons to escape the electron source or emitter. An electric potential is applied between the cathode and the anode in order to accelerate the emitted electrons toward the annular focal track of the anode. X-rays are generated by a portion of the highly accelerated electrons striking the annular focal track.
In order to produce high-quality x-ray images it is generally desirable to maximize x-ray flux, i.e., the number of x-ray photons emitted per unit time. An intense x-ray beam is useful for collecting high-contrast images in as short a period of time as possible. X-ray flux can be increased by increasing the number of electrons emitted by the emitter that impinge on the annular focal track. The flow of electrons from the cathode to the anode transports large amounts of energy to the anode. Target power is a measure of the energy transmitted to the anode per unit time and depends on the electron flux, i.e., the number of electrons emitted per unit time.
The majority of the energy transported to the anode takes the form of thermal energy, or heat. The thermal energy raises the temperature of the annular focal track and the outer perimeter of the target. The anode is rotated to spread the thermal energy around the outer perimeter of the target, allowing for greater target power loads than in x-ray tubes with non-rotating anodes. However, the maximum amount of heat per unit time that the anode can safely handle without being damaged, also referred to as the anode heat input rate capability or anode ratability, limits the maximum target power, and consequently the maximum electron flux and maximum x-ray flux of the x-ray tube.
The anode ratability can be increased by increasing the rotational speed of the anode. However, the rotation of the anode creates stress in the anode. More particularly, the rotation of the anode and the high temperatures at the annular focal track and outer perimeter of the target cause the annular focal track and outer perimeter of the target to radially expand. The radial expansion of the annular focal track and outer perimeter of the target create stresses in the anode that can damage the anode. Higher rotational speeds and higher temperatures result in greater radial expansion and larger stresses. Larger stresses reduce the lifetime of the anode.
Further, where the target comprises a solid disk, thermal energy at the outer perimeter of the target has a 360° thermal conductive path to the anode hub. As a result, a large portion of the thermal energy at the outer perimeter of the target is conductively transferred to the hub and raises the temperature of the hub. In high-power x-ray tubes with solid-disk-type anodes, the temperature of the focal track and outer perimeter of the target can reach 1200° C. or higher, while the temperature of the anode hub may be only a few degrees cooler at around 1100° C. Some approaches that have been taken to prevent overheating and damaging the bearing assembly or otherwise reducing the lifetime of the bearing assembly may result in magnification of load imbalances of the anode on the bearing assembly and thus decrease the useful life of the bearing assembly.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one technology area where some embodiments described herein may be practiced